Near-ir light (700-900 nm) penetrates several centimeters into tissues and passes through bony structures and fat layers. The major contribution to light absorption in this spectral region arises from hemoglobin. In response to tissue metabolism, the hemoglobi oxygenation and its spectral properties change and the hemoglobin concentratio increases. During the previous granting period, we developed frequency domain methods that allowed us to precisely quantify tissue oxygenation changes, in conjunction with measurements at different locations on the tissue surface. High-frequency (~100 MHz) modulated light is applied to a part of the tissue using optical fibers and then collected at different locations, generally 3-4 cm from the source, after the light has diffused deep into the tissue. The determination of the tissue optical properties was very fast and suggested the possibility of performing functional studies in the brain and the muscles that were not available before. One apparent limitation of the use of the diffusive part of the light that propagates in tissues is the relatively low spatial resolution (about 1 cm). However, the applicants reported research showing tha if they observe changes of tissue optical parameters, then the spatial and temporal resolution can be greatly improved with respect to steady-state measurements. The applicants proposed to develop a new detector for the study of rapid changes (1 to 10 ms time scale) of optical parameters deep (1 to 2 cm in tissues. The new detector can localize changes occurring in a small volume (3 mm in radius). It has low noise and relatively high bandwidth. Using this new detector, the applicants proposed to study the dynamics of tissues, particularly the muscle and the brain in the frequency bandwidth up to about 100 Hz. Their approach signals a significant advance with respect to current methodologies and this method could provide physiologists with a new tool for functional studies of the brain and other tissues.